Add support for recording Gauges. (#1344)

* Add support for Gauges in OpenCensus.

* Minor comment fix.

2 files changed, 77 insertions, 0 deletions

diff --git a/api/src/main/java/io/opencensus/common/ToDoubleFunction.java b/api/src/main/java/io/opencensus/common/ToDoubleFunction.java
new file mode 100644
index 00000000..479270f1
--- /dev/null
+++ b/api/src/main/java/io/opencensus/common/ToDoubleFunction.java
@@ -0,0 +1,39 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+// TODO(bdrutu): Change @code to @link when metrics is moved in the API.
+
+/**
+ * Represents a function that produces a double-valued result. See {@code
+ * io.opencensus.metrics.MetricRegistry} for an example of its use.
+ *
+ * <p>Note: This class is based on the java.util.ToDoubleFunction class added in Java 1.8. We cannot
+ * use the Function from Java 1.8 because this library is Java 1.6 compatible.
+ *
+ * @since 0.16
+ */
+public interface ToDoubleFunction<T> {
+
+  /**
+   * Applies this function to the given argument.
+   *
+   * @param value the function argument.
+   * @return the function result.
+   */
+  double applyAsDouble(T value);
+}
diff --git a/api/src/main/java/io/opencensus/common/ToLongFunction.java b/api/src/main/java/io/opencensus/common/ToLongFunction.java
new file mode 100644
index 00000000..9a6e9b19
--- /dev/null
+++ b/api/src/main/java/io/opencensus/common/ToLongFunction.java
@@ -0,0 +1,38 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+// TODO(bdrutu): Change @code to @link when metrics is moved in the API.
+
+/**
+ * Represents a function that produces a long-valued result. See {@code
+ * io.opencensus.metrics.MetricRegistry} for an example of its use.
+ *
+ * <p>Note: This class is based on the java.util.ToLongFunction class added in Java 1.8. We cannot
+ * use the Function from Java 1.8 because this library is Java 1.6 compatible.
+ *
+ * @since 0.16
+ */
+public interface ToLongFunction<T> {
+  /**
+   * Applies this function to the given argument.
+   *
+   * @param value the function argument.
+   * @return the function result.
+   */
+  long applyAsLong(T value);
+}